Q235b carbon steel, main material of early alkaline radioactive liquid waste tank, can be contaminated by 90Sr and 137Cs. In this work, batch contamination experiments were conducted to simulate contamination of steel. The results indicated that Fe3O4, main corrosion product, fell off to form sludges. Furthermore, Sr existed mainly in form of SrCO3 in oxide layer and sludge, and the corrosion made Sr enter into corrosion defects. While Cs was more limited to surface, mainly in form of CsNO3 on the surface of oxide layer and sludge. The findings of this work will facilitate the development of efficient decontamination techniques.Graphic abstract: